gtr,junit: Move Benchmark duration calculation to gtr package

gtr/gtr.go

@@ -103,6 +103,12 @@ type Benchmark struct {
 	AllocsPerOp int64
 }
 
+// ApproximateDuration returns the duration calculated by multiplying the
+// iterations and average time per iteration (NsPerOp).
+func (b Benchmark) ApproximateDuration() time.Duration {
+	return time.Duration(b.NsPerOp) * time.Duration(b.Iterations)
+}
+
 // Error contains details of a build or runtime error.
 type Error struct {
 	ID       int

junit/junit.go

@@ -235,7 +235,7 @@ func createTestcaseForBenchmark(pkgName string, bm gtr.Benchmark) Testcase {
 	tc := Testcase{
 		Classname: pkgName,
 		Name:      bm.Name,
-		Time:      formatBenchmarkTime(time.Duration(bm.NsPerOp) * time.Duration(bm.Iterations)),
+		Time:      formatBenchmarkTime(bm.ApproximateDuration()),
 	}
 
 	if bm.Result == gtr.Fail {